Ebola Vaccine
Background Information The World Health Organization declared the epidemic status of the Ebola virus disease (EVD) in West Africa in August 2014. There have been more than 15,000 cases of EVD with 5,420 plus deaths since it officially became an epidemic. The majority of the cases are in Guinea, Liberia, and Sierra Leone. This epidemic has accelerated research for a possible vaccine. EVD is a filovirus, which belongs to the Filoviridae virus family, which causes severe hemorrhagic fever in humans and nonhuman primates (3). This virus is a negative-strand RNA virus that’s 19-kb in length and codes for 7 viral proteins. One protein, the surface glycoprotein, is responsible for viral entry into host cells. The antigen for this glycoprotein has been the major target in vaccine studies. The chimpanzee adenovirus type 3 (cAd3) vaccine developed by Ledgerwood et al. is a genetically engineered vector that can express wild-type glycoprotein antigens from two species of ebolavirus, the Zaire and Sudan species. These antigens trigger immune responses against the Zaire and Sudan ebolavirus species. The cAd3 Ebola vaccine (cAd3-EBO) phase 1 clinical trial was pushed up due to the urgency of the EVD epidemic. Little is known regarding the exact genetic manipulations of the adenovirus in this study and the mechanism against ebolavirus on Google Scholar or PubMed. The section below details possible methods that might have been used in this study. Genetics and Genomics Adenovirus The adenovirus genome consists of about 36Kb of linear, double-stranded DNA. Adenovirus transcription occurs in two phases - early and late phases, which are before and after replication, respectively. Genes in early-phase transcription include E1, E2, E3, and E4. E1 genes (E1A and E1B) encode for proteins responsible for viral replication. E2 genes encode for proteins involved in the machinery behind viral DNA synthesis and transcription of later genes. E3 genes encode proteins involved in regulation of the infected cells' immune response. E4 genes encodes proteins responsible for metabolism of viral mRNA, promotion of viral DNA synthesis, and turning off host protein synthesis. Somatic gene therapy has been utilizing adenoviruses for some time. These are eukaryotic DNA viruses that can be modified to deliver a transgene to various cell types. Recombinant adenoviruses can deliver an extremely high amount of transgene to almost every cell type. This makes it extremely popular as a delivery method for vaccines. Deleting the E1, E2a, E3, and/or E4 gene can inactive chimpanzee adenovirus vectors. A combination of deletions of these genes may be used. A transgene, which in this case is the surface glycoprotein contained in ebolavirus, is inserted into the chimpanzee adenovirus type 3 vector. This inserted sequence does not cause EVD because the regions for replication and genes encoding toxic proteins have been removed. This eliminates the possibility of the host being infected with ebolavirus. Once the vaccine is introduced into the human body, the surface glycoprotein will be over-expressed. This abundant antigen expression stimulates immune response by generating new antibodies against ebolavirus, protecting vaccinated individual from developing EVD if exposed to ebolavirus in the future. Ebolavirus Each ebolavirus virion consists of a single-stranded, linear negative-sense RNA strand that is usually 18,959 to 18.961 nucleotides long. The 5' end is uncapped and the 3' end is not polyadenylated. The genome consists of genes coding for 7 structural proteins and 1 non-structural protein, which is in the following order: 3' - leader - NP - VP35 - VP40 - GP/sGP - VP30 - VP24 - L-trailer - 5'. NP, VP35, and L genes are found to be endogenous to the genomes of small mammals. The leader and trailer regions are non-transcribed and carry vital information for the transcription and replication regulation of new virions. (6) As with other RNA-coded viruses, ebolavirus shows a high mutation rate within a person and in the general human population reservoir. The approximate mutation rate is 2.0 x 10-3 substitutions per site in a year, which is about the same rate as that of the seasonal influenza virus. This is an immense challenge for vaccine development and methods to control the spread of ebolavirus. Little literature on how the ebola virus could be inactivated was found on PubMed and Google Scholar, but here are the highlights for how the ebola virus could be inactivated. In 2007, Warfield et al. found a novel inactivation technique for ZEBOV or Zaire ebolavirus, which involves use of photoinduced alkylating probe 1,5-iodonaphthylazide (INA). Before this discovery, inactivating filoviruses like ebolavirus involved using high radiation or formalin treatment, which would cause structural disturbances that led to inadequate immune responses. INA treatment of ZEBOV was successful because it completely eliminated the infectious nature of the ZEBOV cells. It works by 1) incorporation into the lipid bilayers of the cell, 2) activation by UV radiation, and 3) alkylation of proteins in the transmembrane domains to inactivate them while maintaining external viral integrity. INA-inactivated ZEBOV vaccine has been successful in preclinical trials and holds much promise for future development. (5) cAD3-EBO Vaccine Phase I Clinical Trial Methods Ledgerwood et al.'s clinical trial of the cAD3-EBO vaccine was conducted in healthy adults ages 18-50 at the National Institute of Health Clinical Center. One single dose of the vaccine was injected intramuscularly in each subject. Two doses were given to the participants, a dose of 2x1010 particle units for group 1 and 2x1011 particle units in group 2. How each group was divided is summarized in Figure 1 on the right. Vaccine Safety Initial data for the cAD3-EBO vaccine show its safety and immunogenicity after a single dose administration of it. Local and systemic side effects occurred, which included fever at higher doses. These effects are similar to studies in other adenovirus vectors. Antibodies generated from the vector were detected in 3 of the 20 participants. Immune responses induced by this vaccine were similar to studies in non-human primates. Ongoing Studies and Future Direction cAD3-EBO vaccine was a bivalent study composed of Zaire and Sudan glycoproteins, but a monovalent vaccine is being developed since the Zaire species is responsible for the current (2014) epidemic. This is called cAd3-EBOZ or EBO Zaire vaccine. Preclinical trials have found this to be more effective and it’s now undergoing phase I clinical trials in quite a number of countries. A booster vaccine called MVA-EBO will soon be evaluated in nonhuman primates who have been primed with the cAd3-EBO vaccine. The high mutation rate of ebolavirus should be taken into consideration. Experience in influenza vaccine development might be adapted for ebolavirus vaccine in the future since they both share a high mutation trait. A long-term approach is necessary to prevent future Ebola epidemics based on all current studies and clinical trials on ebolavirus as well as influenza vaccine. References 1. Ledgerwood, J. E et al. "Chimpanzee Adenovirus Vector Ebola Vaccine - Preliminary Report." New England Journal of Medicine (n.d.): n. pag. 26 Nov. 2014. Web. 7 Dec. 2014. PMID: 25426834. 2. Bausch, D. G. "One Step Closer to an Ebola Virus Vaccine." New England Journal of Medicine (n.d.): n. pag. 26 Nov. 2014. Web. 7 Dec. 2014. PMID: 25426836. 3. CDC article: Filoviridae. Date accessed: Dec 6, 2014. 4. "Patent US6083716 - Chimpanzee Adenovirus Vectors." Google Patent Search. N.p., 4 July 2000. Web. 07 Dec. 2014. 5. Warfield, Kelly L., Dana L. Swenson, Gene G. Olinger, Warren V. Kalina, Mathias Viard, Mohamed Aitichou, Xiaoli Chi, Sofi Ibrahim, Robert Blumenthal, Yossef Raviv, Sina Bavari, and M. Javad Aman. "Ebola Virus Inactivation with Preservation of Antigenic and Structural Integrity by a Photoinducible Alkylating Agent." The Journal of Infectious Diseases 196.S2 (2007): S276-283. 15 Nov. 2007. Web. 7 Dec. 2014. PMID: 17940961.